In EDM processing, a potential difference, i.e. voltage, is applied between a moving wire and the material being machined. The voltage is increased to an amount sufficient to overcome the dielectric breakdown strength of the medium, e.g. deionized water or other dielectric fluid, which separates the wire and the material. When the voltage overcomes the dielectric strength of the medium, a spark is generated, and current flows from the wire to the material or vise versa, depending upon the polarity. Two things result from the current flow. First, the current ionizes the medium between the wire and the material being machined at the point at which the electric field intensity is a maximum and creates a cylindrical envelope of ionized gas, thereby electrically coupling the wire to the material. Second the gas expands in the medium because of the associated elevated temperature and then collapses, much as a cavitation bubble would collapse. The electric energy in the spark, therefore, initially heats to a melting temperature, a very small spot on the surface of the material being machined, and this molten material is then rapidly ejected by the collapsing force of the ionized gas bubble.
Perhaps this is a somewhat different method of defining the mechanics of electric discharge machining. One can imagine, for example, that if an electric current can be developed through an ionized gas, as occurs in tungsten inert gas welding, then virtually all the current supplied is utilized to heat and melt the material away or contribute to bonding, such as in welding. In this case, the material tends to remain at its original site, although it may become molten. In order to remove such material it is necessary to use other mechanical means.
In the case of electric discharge machining, the action of the expanding gas and subsequent collapse of the gas bubble provides the mechanical means for ejecting this micro (very small) puddle of molten metal at the site of the spark initiation.
By-products of the process comprise numerous impurities which may include, among other things, fine particles of the material machined, and metal hydroxides or metal hydroxide salts formed when the medium is ionized. The exact composition of these impurities is not known with certainty. Somethings are, however, known. The principle one of which is that much of the debris is conductive, so that its random distribution in the medium, in effect, alters the distance between the wire and the material being cut, thus reducing the voltage at which dielectric breakdown will occur and thereby limiting the amount of energy that can be carried by the spark. Thus, the impurities effect the dielectric strength of the medium. These impurities, which build up with time during operation, reduce the medium's effectiveness as a dielectric. Thus, it is necessary to filter out these impurities so that the EDM process can be effectively controlled.
In U.S. Pat. No. 4,740,315, a filtering system for an EDM process is disclosed. Therein, an edge filter is utilized to remove the impurities from the EDM process effluent. The impurity coated filter elements, however, are cleaned by air drying the impurities to a powder. At the time that method was proposed, it was not believed that water back flushing would cleanse the filter elements of the impurities because of the tenacity with which the impurities (in sludge form) adhere to the filter elements. This air dry cleansing method is not only time consuming, but it is expensive because the air blown across the elements is typically heated. U.S. Pat. No. 4,740,315 is incorporated herein by reference.